1. Field of the Invention
The present invention relates to a system for connecting a flowline to subsea hardware, and more particularly for remotely connecting a flowline to a wellhead connection or other subsea structure.
2. Brief Description of the Related Art
Flowlines used to connect subsea well completions or subsea manifolds to a production facility are generally deployed to the seafloor independently of other hardware. During the well tie-in phase the flowlines must be pulled from their laydown position to the subsea tree and then connected to the tree. This operation has traditionally been accomplished by divers using a combination of rigging and gantries on the subsea structure, although in deeper water a number of diverless connections have been achieved.
There are three main methods for performing a diverless flowline connection as follows:
(1) Surface Connection/Lay Away
The flowlines are passed from the lay vessel to the drill rig and connected to the tree on the surface, and leak tested. The tree is then "run" down the guidewires and established on the wellhead. The lay vessel lays the flowline away from the tree allowing the other end of the flowline to be connected on the surface.
(2) Subsea Connection/Lay Away
The tree is established on the wellhead by the drill rig. The flowlines are passed from the lay vessel to the drill rig. The flowline ends are then run down guide wires on the tree to align the pipe flange with the tree flowline connection hubs. A flowline connection running tool is then deployed down the guidewires and operated hydraulically to perform the connection subsea.
(3) Laydown/Subsea Connection
Remote pull-in and connection methods are used to complete the flowline connection. This is generally performed after the flowline has been laid on the seabed.
Methods (1) and (2) have a reasonable track record, however, they both suffer the disadvantage of relying upon the drillship and flowline lay vessel being in position in the field at the same time. For multiple well developments this is generally a cost exposure that is uneconomical. Method (2) also requires a large equipment/support area on the subsea structure which complicates the design and increases costs.
In method (3) the flowline is first laid on the seabed to within a short distance, typically to within fifty meters of the wellhead, and then the end of the flowline is pulled along the seabed and engaged to the wellhead connection. The flowline, although flexible, is relatively rigid at the end thereof. Thus, it is necessary that the end of the flowline be brought into proximity with the wellhead connection at the correct angle so that accurate engagement can take place.
Australian Patent Application Serial No. 28509/92 titled "Flowline Connection System," published May 20, 1993, discloses a module for connection to a remotely operated vehicle (ROV). The module attaches to the end of the flowline in a predetermined direction. A single winch system is mounted to the module and includes a winch line for attachment to the wellhead connector so that as the winch pulls in the winch line, the module is pulled towards the wellhead. The module includes a lateral thruster to laterally adjust the position of the module.
It is desirable to have a diverless flowline connection system adapted for use by either mounting the apparatus to the flowline and pulling the flowline to the connector or attaching the apparatus to the subsea manifold/template and winching the flowline to the connector. It is also desirable to have a winching system which allows for a balanced pull force through the centerline of the flowline and which will not put a moment on the flowline during final pull-in. Additionally, it is desirable to have a winch system which permits controlled application of a moment on the flowline to assist in the alignment of the flowline with the connector on the manifold/template. It is also desirable that the apparatus be modular to accommodate various flowline diameters, be light-weight, and include automatic release mechanisms upon power failure.